1. Technical Field
The present invention relates to holders for planar workpieces such as semiconductor wafers and to providing non-contact support for flexible workpieces without distorting or damaging of the workpiece.
2. Description of Related Art
Many manufacturing processes require a holder that holds a workpiece in position for processing. In particular, the processing that forms integrated circuits requires a holder that holds a semiconductor wafer without impeding the processing steps directed at an exposed surface of tie wafer. Safe holding of the wafer is more difficult if the wafer is particularly thin or fragile. In particular, many devices such as cellular telephones, smart cards, and the like require integrated circuits that are thin, e.g., less than about 50 xcexcm. Fabricating the integrated circuits directly on thin wafers can be difficult because the thin wafers are prone to distortion, non-uniform processing, and even rupturing, during the processes required for integrated circuit fabrication. Accordingly, a typical process for manufacturing thin integrated circuit chips fabricates integrated circuits on the front of a wafer and then etches the back of the wafer to thin the wafer after fabrication of the integrated circuits is complete. Plasma etching is a typical method for wafer thinning. For economy of language herein, the xe2x80x9cbackxe2x80x9d of a wafer refers to the surface of the wafer undergoing (or destined to undergo) the etching that thins the wafer. The xe2x80x9cfacexe2x80x9d of the wafer denotes the surface of the wafer not undergoing etching, typically having the integrated circuits thereon. Accordingly, the face of a wafer lies in close proximity to the wafer holder during wafer thinning.
Successful wafer thinning presents several challenges. One challenge is the thickness of the wafer, which is (or soon becomes) very thin, rendering the wafer susceptible to distortion. Distortion generally leads to non-uniform etching, non-uniform heating of the wafer (as during plasma etching, for example) and potential damage to the integrated circuits lying on the face of the wafer. Thus, a wafer holder must be able to hold a thin, easily distorted wafer in a flat position during etching. Of course, the wafer holder must not contact the exposed integrated circuits lying on the face of the wafer next to the wafer holder. Thus, non-contact support for a flexible wafer is needed.
Another challenge for the wafer holder is preventing reactants and debris, e.g., from the etching that thins the wafer, from damaging or contaminating the integrated circuits on the face of the wafer. Non-contact support implies a gap between the wafer and the holder. The holder clearly needs to keep etching reactants and debris out of this gap. More stringently, the wafer holder should protect the thin edge of the wafer from damage and contamination. While not as serious a problem as protecting the face of the wafer, edge contamination may lead to defects in the integrated circuits lying on the contaminated regions of the edge.
Siniaguine and Steinberg (PCT International Publication No. WO/97/45862, hereinafter xe2x80x9c""45862xe2x80x9d) describe a non-contact holder for wafer-like objects in which a vortex of rotating air provides both the vacuum support for the wafer while providing the air cushion ensuring non-contact with the wafer holder.
The present invention relates to a non-contact holder for flexible workpieces and is particularly suited for holding thin workpieces without substantial distortion. One embodiment of the present invention is a chuck having one or more tangential orifice on a curved surface of a cavity. The cavity opens to a surface of the chuck that is adjacent a workpiece held by the chuck. The tangential orifice introduces a gas flow into the cavity in a direction having a component tangential to the curved surface of the cavity. This gas flow creates of a vortex within the cavity, and the vortex creates vacuum attraction to hold a workpiece in close proximity to the chuck. Gas exiting from the cavity prevents contact between wafer and chuck. Additionally, one or more central orifice on a top portion of the cavity of the chuck introduces gas to reduce the vacuum attraction at the center of the chuck. The central orifice can introduce gas along the central axis of the cavity or offset from and/or at an angel with the central axis. The chuck with the central and tangential orifices has the advantage of applying a more uniform holding force to the workpiece thereby causing less distortion or damage of the workpiece.
Many small diameter chucks in accordance with the invention can be located close together to help avoid distortion or damage when holding very thin wafers for processing. Each chuck has a preferential path for the exiting gas according to the location of the tangential orifice, and the combination of the gas exiting from the chucks provides a gas flow that resists entry of debris or reactants between the workpiece. The gas flow from the central orifice widens the flow of exiting gas from each chuck, and allows more freedom in the number and positioning of chucks to provide a more uniform gas flow around the perimeter of the workpiece.
One specific embodiment of the invention is a chuck that includes a cavity, a first orifice, and a second orifice. The cavity opens at a surface that is adjacent a workpiece when the chuck holds the workpiece. The first orifice directs a first gas flow into the cavity, and the first gas flow creates a vortex having a low-pressure region. The second orifice directs a second gas flow into the low-pressure region of the vortex. The combination of the first and second gas flows creates a more uniform pressure profile under the chuck and thereby reduces distortion of a workpiece held in the chuck.
Another embodiment of the invention is a workpiece holder that includes a plurality of chucks. Each chuck includes a cavity, a first orifice, and a second orifice. The cavities of the chucks open at a surface that is adjacent a workpiece when the holder holds the workpiece. The first orifice in each chuck directs a first gas flow into the cavity, and the first gas flow creates a vortex having a low-pressure region that attracts the workpiece to the surface of the holder. The second orifice directs a second gas flow into the low-pressure region of the vortex. Gas exiting from chucks fans out around the perimeter of the workpiece to provide a uniform gas flow that protects a face of the workpiece from debris and reactants that might otherwise enter the space between the holder and the workpiece.